Cytokinins are plant hormones that regulate a large number of developmental and physiological processes in plants. With the exception of diphenylurea, all naturally occurring cytokinins are 6-substituted purine drivatives that can be further modified by substitution. Cytokinin oxidase/dehydrogenase (CKX) is an enzyme that is responsible for cytokinin degradation. The modulation of cytokinin levels by either exogenous application of cytokinins or regulation of their endogenous levels genetically through cytokinin oxidase/dehydrogenase (CKX, EC 1.5.99.12) have already shown possible applications in agriculture. CKX inhibitors are cytokinin derivatives that can delay cytokinin degradation and therefore prolong the period of cytokinin action. For example, exogenous application of cytokinins led to shortening of the time to anthesis in tomato (Sawhney and Shukla, Am J Bot 81:1640, 1994) or reversion of male sterility in barley (Ahokas, Proc. Natl. Acad. Sci. USA 79:7605, 1992). Anther- and pollen-specific expression of CKX in maize was shown to be a potential tool for generating male sterility for production of hybrid varieties of traditionally non-hybrid crops. Recent work reported CKX involvement in regulation of rice grain production (Ashikari et al., Science 309:741, 2005).
We have recently discovered that novel generations of CKX inhibitors could be based on 2-substituted 6-anilinopurines (CZ302225B6). It is an object of the present invention to provide further compounds having improved selectivity and efficiency index in plants, i.e., having lower toxicity and higher activity than known active substances. The compounds of this invention exhibit surprisingly strong antistress and antisenescence properties on plants.